Method for obtaining single crystal ferrite films



March 3, 1970 R. J. GAMBINO 3,

METHOD FOR OBTAINING SINGLE CRYSTAL FERRITE FILMS Filed April 25, 1966FIG. 1

FERRITE 2 SINGLE CRYSTAL MAGNESIUM OXIDE INVENTOR. RICHARD J. GAMBINOUnited States Patent 3,498,836 METHOD FOR OBTAINING SINGLE CRYSTALFERRITE FILMS Richard J. Gambino, Yorktown Heights, N.Y., assignor toInternational Business Machines Corporation, Armonk, N .Y., acorporation of New York Filed Apr. 25, 1966, Ser. No. 544,788 Int. Cl.G03g 19/00; H01f 1/34 US. Cl. 117-237 14 Claims ABSTRACT OF THEDISCLOSURE A method for epitaxially depositing a single crystal ferritefilm on a single crystal substrate is disclosed. Epitaxial deposition ofa ferrite having a spinel structure is accomplished by heating asolution of a polymetallic ferrite and a flux on the surface of a singlecrystal substrate by heating the solution to a temperature sufficient tovolatilize the flux completely thereby leaving a single crystal ferritefilm residue.

Transparent films of magnetic materials are of current interest becauseof their utility as magneto-optical memory devices. In the usual case,the variation of polarization of an incident light beam with variationin magnetic field or with localized changes in the magnetic state of thefilms is used to obtain outputs indicative of stored information.

Magneto-optical films and methods of manufacture are well known in theprior art. 'Methods of manufacture of these films include suchtechniques as vacuum deposition and the chemical reduction ofappropriate materials. Most of the known techniques do not provideepitaxial single crystal films which are particularly desirable in thatbetter optical transmission characteristics can be attained. Prior artvapor deposition techniques, for example, usually producepolycrystalline films in which grain boundaries affect both the opticaland magnetic properties of the resulting films. Prior art techniques arealso limited in the size of films obtainable which possess the desiredsingle crystal characteristic. Where single crystal films of desiredsurface area and of sufficient thinness to transmit light are desired,they must be cut and ground from bulk single crystals prior to use. Thestrains, cracks and other defects introduced in such thinning operationusually degrade the crystals magneto-optical properties.

It is, therefore, an object of this invention to provide transparentepitaxial thin films of spinel ferrites.

Another object is to provide transparent epitaxial thin films of spinelferrites which have improved optical transmission characteristics.

Another object is to provide a method of growing single crystalepitaxial films by a volatile flux growth process.

Another object is to provide a method of growing single crystalepitaxial ferrite films in which the size of the resulting films islimited only by the surface area of the seed upon which growth occurs.

Still another object is to provide a method of growing single crystalepitaxial ferrite films in which the thickness of the resulting films iscontrollable.

Still another object is to provide a method of growing single crystalferrite films which is both inexpensive and reproducible.

Yet another object is to provide single crystal ferrite films which aretransparent, magnetic in nature and suitable for use in magneto-opticalmemory devices.

In accordance with the present invention, a method for preparingepitaxial single crystal films of spinel ferrites selected from thegroup consisting of lithium, lithium chromium, nickel zinc, nickel,magnesium and cobalt ferrites is provided. Epitaxial deposition isaccomplished from a solution of a polymetallic ferrite having a spinelstructure and flux by heating the solution at a temperature sufficientto melt the flux. Briefly, the method includes the steps of providing aseed crystal of magnesium oxide or other suitable material which istransparent and noncontaminating to the resulting ferrite film. One ofthe ferrites from the above mentioned group is mixed with a flux such:as anhydrous sodium or lithium carbonate and with isopropyl alcohol toform a slurry. The slurry is applied to the surface of the seed crystaland air dried to remove the alcohol solvent. The seed crystal orsubstrate is then placed in a platinum dish and introduced into apre-heated furnace for firing. The substrate and mixture are then heatedfor 20 hours in air or oxygen at a temperature Slll'fiClCl'lt todissolve the ferrite in the flux and to cause crystallization of theferrite by volatilization of the flux. The seed, after heating, iscovered with a single crystal epitaxial film of ferrite 5 to 50 micronsthick.

The foregoing and other objects and features of the present inventionwill become more apparent when taken in conjunction with theaccompanying specification and drawings in which FIG. 1 is a perspectiveview of a single crystal epitaxial film of spinel ferrites disposed onthe surface of a single crystal substrate.

FIG. 2 is a diagrammatic drawing showing a magnetooptical systemutilizing a single crystal epitaxial film of a spinel ferrite.

Referring now to FIG. 1, there is shown a perspective view of asubstrate 1 having an epitaxial single crystal film 2 of a spinelferrite disposed on a surface thereof. Substrate 1 has thecharacteristic that it is transparent to the wave lengths of lightusually used in conjunction with magneto-optical devices. Substrate 1also has the characteristic that it is a single crystal material which,in effect, forms the pattern for subsequently applied layers ofmaterial. By providing the single crystal substrate, subsequently formedfilms will be epitaxial i.e., they will conform to the single crystalpattern of the substrate. In addition to the above mentionedcharacteristics, the substrate should be of such a nature that it isnoncontaminating to the finally formed epitaxial film. One substratewhich fulfills all the above criteria is magnesium oxide. Crystals ofaluminum oxide and nickel oxide while not as satisfactory as crystals ofmagnesium oxide, have been successfully used.

In forming film 2, a slurry is first formed by mixing equal parts byweight of a flux such as sodium or lithium carbonate together with aspinel ferrite such as lithium, lithium chromium, nickel zinc, nickelmagnesium or cobalt ferrite in isopropyl alcohol. The mixture is thenground in a mortar until homogenous and air dried until a powderedmixture forms. Alcohol is then added to the mixture which is stirred toform a slurry having the consistency of light cream.

Prior to the actual preparation of the film, the surface area of thebest large face of the magnesium oxide substrate is measured, rinsedwith alcohol, dried and weighed. To form the epitaxial film, thesubstrate is placed on a fiat surface with the best face upward. Theslurry of ferrite and flux is applied to the substrate crystal face froma medicine dropper, three or four drops at a time and then air dried forat least 15 minutes after which the substrate is placed on a hot platefor 5 to 10 minutes. A thin homogeneous coating of ferrite and fluxcovers a face of the substrate using the above steps. The ferrite andcoating are then weighted to determine if sufficient ferrite has beenapplied to the surface to give a film of the desired thickness. Ifthicker coatings are desired, the application and heat treating of theslurry should be repeated until a coating of desired weight is obtained.

With a coating of desired Weight on the substrate, the substrate isplaced in a platinum boat and introduced into a furnace which has beenpre-heated to approximately 1200 C. The coated substrate is then heatedin air or oxygen for approximately 20 hours.

After firing for the desired time, the boat is withdrawn and thesubstrate face is coated With an epitaxial single crystal of a spinelferrite.

It should be appreciated that the constituents making up any of theferrites used may be obtained separately and fired to provide thedesired ferrite rather than starting with the reacted ferrites as hasbeen suggested above.

The mechanism which produces the resulting epitaxial film is one ofvolatilized flux growth. When the substrate and coating are introducedinto the furnace, the flux first becomes molten at a temperature ofabout 700 C. As the substrate heats up to the furnace temperature, theferrite dissolves in the flux forming a flux-ferrite solution which wetsthe surface of the substrate to form a liquid layer of uniformthickness. In the molten state, a convective stirring occurs within thematerial such that the flux, which must be less dense than the ferrite,is displaced to the topmost portion of the molten material. The fluxthen gradually volatilizes leaving behind a supersaturated solution offerrite in the molten flux. As the flux volatilizes further, thesuper-saturated solution sinks to the bottom of the molten material anddeposits ferrite on the face of the substrate which conforms to thecrystalline structure of the substrate and results in an epitaxial filmof spinel ferrite having a thickness of to 50 microns depending on theinitial Weight of ferrite per unit area of substrate.

The following are specific examples of the method of preparing epitaxialthin films of spinel ferrites using different ferrites and fluxes.

EXAMPLE 1 A substrate of magnesium oxide which is transparent to desiredlight wavelengths is provided. The substrate has a face which isoriented in one of the major crystallographic planes i.e. (111) (100). A(100) plane can be obtained by cleaving the crystal. The substrate isprepared by rinsing with isopropyl alcohol and drying. A mixturecontaining equal parts by Weight of anhydrous sodium carbonate and alithium chromium ferrite having the formula 0.5 2.5-x x 4 where:

is mixed in alcohol by grinding in a mortar to form an homogenousslurry. Typical weights of flux and ferrite to form the mixture are 0.2gm. of each. The slurry is air dried to form a powder and re-mixed withsufficient alcohol to form a slurry having the consistency of lightcream. Droplets of the slurry are then applied to the surface of thesubstrate and air dried for minutes. Further drying is accomplished byheating on a hot plate for 5 to 10 minutes. The slurry may beadvantageously applied by spraying, painting or dipping. Depending onthe ultimate film thickness desired, the step of applying and drying theslurry may be repeated several times.

The weight of ferrite and flux mixture which must be applied to thesubstrate to obtain any desired thickness maybe calculated knowing thedensity of the ferrite, the surface area of the substrate and the Weightpercentage of ferrite in the mixture. If a (20 10 cm.) thick film isrequired on a substrate 1 cm.- in area of a ferrite with a density of 6gm./cm.- then .0120 gm. of ferrite are required which would be obtainedby applying .024 gm. of a 50 wt. percent ferrite-flux mixture.

The weight ratio of flux to ferrite is not critical and may varyconsiderably without affecting the utility of the resulting ferritefilm. The ratio of fiux to ferrite may be varied over the range of 1part of flux to 3 parts of 4 ferrite by weight to 9 parts of flux to 1part of ferrite by weight.

The substrate is then placed in a fiat-bottomed shallow platnum boat andintroduced into a furnace pre-heated to a preferred temperature of 1200C. Firing temperatures in the range of l050 C. to 1300 C. have been usedsuccessively without affecting the quality of the resulting epitaxialfilms. The substrate is heated at the firing temperature (1200 C.) for20 hours in air or oxygen. Periods of 1 hour to 48 hours have also beenused to successively produced epitaxial ferrite films. The thicker theresulting film required, the longer the firing time necessary tovolatilize the flux. After heating, the substrate is cooled to roomtemperature and used with the substrate attached (MgO is transparent) orthe substrate may be removed by grinding or cleaving. The resultingferrite film is from 5 to 50 microns in thickness depending on theweight of ferrite per unit area of substrate of the initially appliedcoatings.

EXAMPLE II The steps used are the same as described in connection withExample I With the exception that anhydrous lithium carbonate is used asa flux instead of anhydrous sodium carbonate.

EXAMPLE III The steps used are the same as in Example I with theexception that the ferrite used has the formula:

The steps used are the same as in Example I with the exception that theferrite used is:

NiFe O EXAMPLE VI The steps used' are the same as in Example II with theexception that the ferrite used is:

NiF'e O.

EXAMPLE VII The steps used are the same as in Example I with theexception that the ferrite used is:

Ni Zn Fe O Where:

EXAMPLE VIII The steps used are the same as in Example II with theexception that the ferrite used is:

Ni ZII F62O4 where:

EXAMPLE IX The steps used are the same as used in Example I with theexception that the ferrite used is:

EXAMPLE X The steps used are the same as used in Example II with theexception that the ferrite used is:

EXAMPLE XI The steps used are the same as used in Example I with theexception that the ferrite used is:

EXAMPLE XII The steps used are the same as used in Example II with theexception that the ferrite used is:

CoFe O In the magneto-optical device shown diagrammatically in FIG. 2 ofthe drawing, a single crystal film of a spinel ferrite 2 on a substrate1 is mounted between spaced cross polarizing filters (i.e., polarizer 3and analyzer 4). The crystal is placed in a magnetic field (e.g., thatproduced by an electromagnet 5 or by Helmoholtz coils). A light source 6and a photocell 7 are placed such that light to which the cell 7 isexposed is that which originated at light source 6 and passessuccessively through polarizer 3, single crystal film 2, substrate 1 andanalyzer 4. Since the degree of rotation of the plane of polarized lightpassing through the single crystal film of spinel ferrite is dependentupon the magnetic field, the amount of rotation of light originating inthe light source and passing through the polarizer, analyzer and thesingle crystal film to the photocell can be varied by varying thestrength of the magnetic field of the magnet. In the ferromagneticregion at magnet saturation, the rotation is independent of the appliedmagnetic field and the maximum rotation can be obtained.

In FIG. 2, a crystal of Li Fe O the manufacture of which has beendescribed in connection with Examples III and IV above, five microns inthickness transmits 34% of the incident light at a wavelength of 0.8microns. The magneto-optical rotation at saturation for this material is1000 of rotation per centimeter of thickness. Thus, a film five micronsthick rotates the plane of polarization O.5. Spinel ferrite films suchas described in the other examples provide similar rotations under thesame conditions.

When spinel ferrite, single crystal films of suflicient thickness aregrown, the supporting substrate can be removed by either cleaving orgrinding it away. While the films of the present invention have beendescribed in conjunction with devices Which utilize Faraday rotation, itshould be appreciated that other uses for such films can be made. Forinstance, a plurality of films laminated together could be utilized toform a magnetic recording head which utilizes only the magneticproperties of the ferromagnetic films.

While the invention has been particularly shown and described withreference to a preferred method, it will be understood by those skilledin the art that various changes in the details may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A method of making single crystal ferrite films comprising the stepof epitaxially depositing from a solution of a polymetallic ferrite anda molten salt solvent a poly metallic ferrite having a spinel structureon a single crystal substrate by heating said solution at a temperaturesufficient to volatilize said molten salt solvent.

2. A method of making single crystal ferrite films according to claim 1wherein the polymetallic ferrite includes at least one ferrite selectedfrom the group consisting of:

where:

NIFC2O4 3. A method of making single crystal ferrite films according toclaim 1 wherein said molten salt solvent is one selected from the groupconsisting of anhydrous sodium and lithium carbonate.

4. A method of making single crystal ferrite films according to claim 1wherein said substrate is a single crystal of magnesium oxide.

5. A method of making single crystal ferrite films according to claim 1wherein said temperature sufficient to volatilize said molten saltsolvent includes the temperature range of 1050" C. to 1300 C.

6. A method of making single crystal ferrite films according to claim 1wherein said temperature sufiicient to volatilize said molten saltsolvent is 1200 C.

7. A method of making single crystal ferrites comprising heating amixture of a ferrite selected from the group consisting of:

and a flux selected from the group consisting of anhydrous sodium andlithium carbonate on a face of a single crystal of magnesium oxide for atime and temperature sufficient to dissolve said ferrite in said fluxand completely volatilize said flux such that a residue of epitaxiallydeposited film remains on said crystal face.

8. A method of making single crystal ferrites according to claim 7wherein said time includes the range of 1 hr. to 48 hrs.

9. A method of making single crystal ferrites according to claim 7wherein said time is 20 hrs.

10. A method according to claim 7 wherein said temperature includes thetemperature range of 1050 C. to 1300 C.

11. A method according to claim 7 wherein said temperature is 1200 C.

12. A method of making single crystal ferrites comprising the steps of:

preparing a slurry consisting of a homogeneous mixture of ferritesselected from the group consisting of:

where and a flux selected from the group consisting of anhydrous sodiumand lithium carbonates and isopropyl alcohol;

providing a single crystal substrate of magnesium oxide, applying atleast one coating of said slurry to a crystal face of said substrate,firing said coated crystal in a furnace in an oxidizing atmosphere in atemperature range of 1050 C. to 1300 C. for 1 to 48 hours to dissolvesaid ferrite in said flux and to volatilize said flux such that a where7 8 ferrite film 5 to 50 microns thick is epitaxially de- 3,332,7967/1967 Kooy 252--62.56 posited on said substrate crystal face. 3,404,02610/1968 Skudera et a1 117169 13. A method accordlng to clalm 12 whereinthe step OTHER REFERENCES of preparing a slurry includes the step ofmixing flux and ferrite over a range of weight ratios of one part fluxto Cech et Pfepafatloll of and C00 y three parts of ferrite by weight tonine parts flux to one tals by Halide Decomposition, TTaTlS- Metals,part of ferrite by weight. vol. 51, pp. 150 to 161.

14. A method according to claim 12 wherein the step of preparing aslurry includes the step of mixing equal WILLIAM MARTIN Pnmary Exammerparts by weight of said ferrite and said flux. 10 B PIANALTO, AssistantExaminer References Cited U.S. C1. X.R.

UNITED STATES PATENTS 117-236; 25262.56, 62.61, 62.64

3,150,925 9/1964 Gambino 252--62.56

3,305,301 2/1967 Remeika 25262.61 15

